Drill tools are used to produce holes or countersinks in a production metalworking environment. There are known drill tools which use a drill insert having cutting surfaces selectively secured to a holder. The drill insert is generally constructed of high speed steel, carbide or other materials as necessitated by the work piece. The drill insert is fit into a corresponding slot in the holder, and is secured there with retaining screws. During a drilling operation, the holder with the drill insert in place is indexed in relation to the part. From this set point, subsequent movements of the machining operation are calculated, such as the location and the depth of the hole to be cut. During the machining operation the drill insert may become dull and replacement may be required. Ideally, the drill tool allows the holder to remain in the machine in an indexed position while the drill insert is replaced. The new drill insert needs to index to the same place within the tool holder as the original insert, and any machining errors in the insert must be minimized.
Ideally the centerline of every drill insert will correspond with the axial centerline of the holder, and the cutting surfaces or point geometry formed precisely relative to this centerline. A change in the index position of the new bit within the slot of the holder can cause significant error. Additionally, the insert itself must be machined with cutting surfaces and an overall outside diameter which must be kept to within very close tolerances to avoid other possible errors when assembled with a tool holder. Of significant importance is maintaining the outside diameter also in high tolerance with respect to the axial centerline of the tool holder. In the methods of manufacturing drill inserts of this type, no effective remedy to errors with respect to overall outside diameter and relative positioning of the cutting surfaces associated with the insert have been found. This in turn causes TIR errors when assembled with a holder and used in a drilling operation. There is therefore a need for a drilling tool, wherein a drill insert can be precisely manufactured to mate with a tool holder in a manner which provides self-indexing and self-alignment of the drill insert with respect to the tool holder.
A variety of methods of attaching a drill insert to the holder have been attempted to properly index the drill insert within the holder. Such attempts include using a locating slot formed on the bottom of the drill insert opposite the cutting edge. The slot mates with a corresponding pin or dowel positioned in the holder. The dowel pin is of a nominal size and must be manufactured within a tolerance limit, and the corresponding slot in the drill insert also has a size tolerance to allow the dowel to be inserted. These manufacturing tolerances cause a clearance to occur between the locating dowel in the holder and the locating slot in the drill insert. This clearance does not promote repeatability in placement of the drill inserts within the holder, and instead adds to the index error as described above.
Other configurations include biasing the insert against one or more seating surfaces, such as by offsetting the retaining screw hole in the tool holder with respect to the corresponding hole in the drill insert. This offset causes a camming action to occur when the retaining screw in tightened forcing the drill insert to be seated against the seating surfaces. Although this may lead to reduced errors in some cases, it is also possible that if the drill insert is positioned in the slot in an orientation which differs from the position of the insert when manufactured, the error could be increased. Though it may be possible to machine the locating slot in the insert to be exactly centered and have very low tolerances on the finished size of the slot, this requires extremely tight tolerances be kept for the placement and dimensions of the locating slot with respect to the centerline of the drill insert. Manufacturing within the tight tolerances required for this indexing system would dramatically increase the price of each drill insert. This would also make use of the insert and assembly holder more difficult, as the insert would have to be precisely positioned to receive the pin, and forced into the proper location about the locating pin.
It therefore would be desirable to have a drill tool assembly which allows a drill insert to be precisely positioned with respect to the tool holder to reduce errors in the positioning of the insert relative to the holder, but which allows simplified and consistent assembly of the insert with the holder. It would also be desirable to provide a method of manufacturing drill inserts for use with a holder which allow the drill insert to be manufactured in a manner to be consistently and precisely positioned with respect to any holder with which it is used, and to minimize errors in such positioning.